AEROSOL OPTICS
Amplification of light within aerosol particles
accelerates in-particle photochemistry
Pablo Corral Arroyo^1 , Grégory David^1 , Peter A. Alpert^2 , Evelyne A. Parmentier^1 ,
Markus Ammann^2 , Ruth Signorell^1 *
Optical confinement (OC) structures the optical field and amplifies light intensity inside atmospheric
aerosol particles, with major consequences for sunlight-driven aerosol chemistry. Although theorized, the
OC-induced spatial structuring has so far defied experimental observation. Here, x-ray spectromicroscopic
imaging complemented by modeling provides direct evidence for OC-induced patterning inside photoactive
particles. Single iron(III)Ðcitrate particles were probed using the iron oxidation state as a photochemical
marker. Based on these results, we predict an overall acceleration of photochemical reactions by a factor of
two to three for most classes of atmospheric aerosol particles. Rotation of free aerosol particles and
intraparticle molecular transport generally accelerate the photochemistry. Given the prevalence of OC
effects, their influence on aerosol particle photochemistry should be considered by atmospheric models.
A
tmospheric aerosol particles are suspen-
sions of solid and liquid particles in air
that influence both climate and air qual-
ity ( 1 ). Aerosol and cloud chemistry play
a crucial role in the processing of atmo-
spheric particulate matter and are key parts of
global atmospheric models ( 2 – 6 ). Chemical re-
actions triggered by sunlight in the gas and
particle phase have been recognized as a major
contributor to the degradation and oxidation
of matter in atmospheric aerosols ( 7 ). Energy-
or charge-transfer reactions driven by triplet
states ( 8 – 10 ), photolysis of nitrate and nitrite
( 11 ), and photolysis of iron carboxylate com-
plexes ( 12 , 13 ) are examples of atmospherically
relevant photochemical processes that involve
the particle phase. Photochemical reactions
can also be promoted at the surface of atmo-
spheric aerosol particles ( 14 – 16 ). It has been
reported that interface effects and surface
charging can cause acceleration of chemical
reactions in microdroplets and nanodroplets
( 14 , 17 – 20 ), observations that have especially
sparked interest in the use of microdroplets
as new powerful reactors for chemical syn-
thesis ( 15 , 16 ).
Here, we report another intriguing phenom-
enon in particle reactions: the influence of
optical confinement (OC) on photochemical
reactions in aerosol particles ( 21 ). OC leads to
spatial structuring of the light intensity inside
the particle [nanofocusing or shadowing; see
supplementary materials (SM) section S1]
( 21 – 24 ) and hence to spatial variations of
photochemical rates. For a photochemical
reaction step, the overall reaction rate,j, in
a particle is given by
j¼
IðÞll
hcφðÞlslðÞ∫en;k;r
→
;l
Cr→
dr
→
ð 1 Þwherer
→
is the position in the particle; the
local light-enhancement factor,e
n;k;r
→
;l
,
is the ratio of the local light intensity,Ip
r
→
,
to the incident light intensity,I;lis the
wavelength of the light;his Planck’s con-
stant;cis the speed of light;nandkare the
real and imaginary parts of the complex index
of refraction, respectively;φis the quantum
yield;sis the molecular absorption cross
section; andC
r→
is the molecular density of
the reactant. Shadowing results from strong
light absorption (highkvalues), reducing the
average light intensity in the particle. Because
shadowing is also present in extended con-
densed systems (referred to as“bulk”), parti-
cle and bulk reaction rates are comparable in
this case ( 21 ). Nanofocusing is a resonancephenomenon that is tied to the spatial con-
finement by particles. It increases the aver-
age light intensity in the particle compared
with the incident intensity—that is, on aver-
age,e>1—thereby accelerating the reaction
(Eq. 1) in particles compared with that in
bulk systems, where nanofocusing does not
occur. The influence ofeon photokinetics
and radiation balance can be substantial,
but atmospheric models do not usually ac-
count for it. The acceleration of photochem-
ical reactions in typical atmospheric aerosol
particles is still awaiting a comprehensive
evaluation. Although basic research has dem-
onstrated the overall acceleration of photo-
chemical reactions in single aerosol droplets
( 21 ), the predicted spatial variation of photo-
chemical rates induced by OC effects has not
been directly observed or quantitatively con-
strained within submicron aerosol particles
until now.
Here, we directly imaged the local compo-
sitional gradients resulting from OC inside
single submicrometer-sized particles (Fig. 1).
Highly viscous, dried Fe(III)–citrate (FeCit)
particles were exposed to ultraviolet (UV) light
(hn, wherenis the photon frequency), result-
ing in the photoreduction to Fe(II):FeIIICit→
hn
FeIIðÞCit• ð 2 ÞScanning transmission x-ray microscopy
coupled with near-edge x-ray absorption fine
structure (STXM-NEXAFS) spectroscopy was
used to image the temporal evolution of the Fe
(III) fraction,a(eq. S2). A quantitative model
proves that OC effects are responsible for theSCIENCEscience.org 15 APRIL 2022•VOL 376 ISSUE 6590 293
(^1) Department of Chemistry and Applied Biosciences, ETH
Zürich, Vladimir-Prelog-Weg 2, CH-8093 Zürich, Switzerland.
(^2) Laboratory of Environmental Chemistry, Paul Scherrer
Institute, 5232 Villigen PSI, Switzerland.
*Corresponding author. Email: [email protected]
Fig. 1. Maps of the Fe(III) fraction
upon UV photoreduction of iron in
submicron FeCit particles.(Ato
F) STXM-NEXAFS experiments [(A) to
(C)] and simulations [(D) to (F)] of
the column-averaged Fe(III) fraction,
ac(see color scale bar).acis shown
before irradiation [(A) and (D)] and
after94min[(B)and(E)]and139min
[(C) and (F)] of irradiation with UV light
in the direction indicated by the blue
arrow.acis averaged along the x-rayÐ
beam propagation direction, which is
perpendicular to the UV-light propagation
direction (fig. S2). The white arrows
indicate the hotspot region that is due
to nanofocusing.
A
0.5 μm
B
C
D
E
F
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
c
0.5 μm
0.2 μm 0.2 μm
0.2 μm 0.2 μm
RESEARCH | REPORTS